Remediation Methods of Crude Oil Contaminated Soil
Authored by Ding Xuezhi,
Crude
oil is a quick and easily accessible source of energy, making our life
comfortable and raising the standards of living. It can be found naturally in
many parts of the world, particularly in the USA, Russia, Romania, Iran,
Mexico, Iraq, Saudi Arabia, Kuwait, Libya, and Nigeria [1]. The petroleum
industries generate billion tons of crude oil, natural gas and its derivatives
every year. All of these are then undergone further processing for the
production of refined products such as diesel, gasoline, petrol and lubricants
[2]. It is recorded by international energy agency that demand of oil all over
the world in 2015 was 97 million barrels/day which is expected to be 100
million barrels/day up to 2021 [3].
Crude
oil is composed of volatile liquid hydrocarbons with varying molecular weight
and structure. It contains more than 17,000 hydrocarbons and its classification
are based on the most prevalent compound present in it. The three main
hydrocarbons components present in crude oil are compiled in Table 1 [4-6].
Crude
oil contamination is one of the major environmental problems effecting aquatic
and terrestrial environments. At present, approximately 80% of lands are
affected by petroleum origin products i.e., hydrocarbons and these products are
used in oil and chemical industries as energy source [7]. Crude oil makes a
covering on the surface of soil and causes the retention of carbon dioxide
produced by soil organisms. It also decreases the soil porosity by sticking the
soil particles together. The amount of loss depends on the amount and grade of
oil spilled [1].
Many
accidental spillages of crude oil have threatened the nature. The largest
accident in the history of mankind that caused environmental disaster is “Gulf
war oil spill” (1991). This accident caused the spill of millions of gallons of
crude oil from destroyed oil wells into the water and surrounding land covering
49 square km of an area [8]. Similarly, “Keystone pipeline accident” (2017) is
another disaster of oil spillage. This spill caused the spread of 210,000
gallons of oil on the grass as well as in the agricultural area at southeast of
the small town of Amherst in northeast South Dakota [9].
Polycyclic
aromatic hydrocarbons (PAH) present in crude oil, declared as primary
environmental pollutant by the United States Environmental Protection Agency
are mutagenic and carcinogenic [10]. A prolonged contact time of stable PAH
with soil stimulate the phenomenon called soil aging, leading to the resistant
of soil to any treatment [11]. Leakage of these contaminants from the soil to
the ground water can pose risk to human health, vegetation and biological
environment [7]. So, it is very important to clean the soil from these harmful
substances to guard life from their deadly effects. Besides, by remediating oil
contaminated sites more land can be available for residence as well as
agricultural activities.
Numerous
countries are developing their own strategies to cope with the soil
contamination done by crude oil e.g., Lebanon, Kuwait and some other middle
east countries have organized oil spill working groups by the aid of
environment research organizations for assessment and future remediation of the
affected areas [2]. Numerous methods for the removal of crude oil from the contaminated
soil have been devised. A quick, nature friendly and cost-effective method is
required for this purpose. This review focuses on the current developments of
some generally accepted remediation techniques used to treat crude oil
contaminated soil.
Chemical
Methods
Chemical
oxidation is an efficient method to remove dangerous wastes from the soil at
the oil spilled sites. The efficiency of this method strongly depends on the
soil matrix. Fenton’s reagent, a mixture of Hydrogen peroxide and Ferric ion,
is used for chemical oxidation. Hydrogen peroxide is a strong oxidizing agent
that generates hydroxyl ions during Fenton’s reaction while ferric ion acts as
catalyst. Hydroxyl ions are very powerful and effective agents that destroy the
contaminants present in the soil [12,13] demonstrated that removal of oil from
sand at lower pH by using Fenton’s reagent is much efficient than at natural pH
or peat.
Another
efficient oxidant that is used for the removal of crude oil from soil is ozone.
It is easy to generate, store and handle for in situ treatment. Polycyclic
aromatic hydrocarbons are more reactive with ozone in comparison o alkanes.
Reactivity of poly aromatic hydrocarbons depends on the number of rings,
heteroatoms presence or absence and alkylation level. Ozone also support
microbial community present in the soil as it generates oxygen on its
degradation, so it can be helpful in bioremediation method to aid microbial
growth [14]. Chemical method is a quick way to treat contaminated soil, but
chemicals may pose a serious threat to the nearby soil and living beings due to
leaching or side reactions.
Physical
Methods
Excavation
of crude oil contaminated soil is the quickest and safe way but not a
sophisticated and cheap method. The contaminated soil is removed and
transported to appropriate landfill for the disposal. The samples are collected
from bottom and sidewalls of the excavated area to check if the site is clean
or not [15-17].
Another
physical method is the washing of contaminated soil. Washing with organic
solvents such as ethanol- water mixture and ethyl acetate-acetone-water mixture
exhibited significant removal of hydrocarbons from the contaminated soil
[18-20]. Soil washing does not only treat the oil contaminated soil but also
remove the heavy metals from the soil. The efficiency of washing can be
enhanced by the addition of surfactants. Studies showed that both artificial
and natural surfactants are helpful in the removal of crude oil. Different
surfactants remove different fractions of crude oil e.g. artificial surfactant
sodium dodecyl sulfate (SDS) removed aliphatic hydrocarbons while natural
surfactants saponin and rhamnolipid removed polycyclic aromatic hydrocarbons from
the contaminated soil [21]. This method no doubt is simple and efficient,
however, it is very prolonged, time consuming and very costly. Transportation
of contaminated soil to disposal site is another big problem. Surfactants might
be dangerous due to their possibility of adhesion to soil particles.
Thermal
Methods
In
Thermal stripping/low temperature thermal desorption/soil roasting contaminated
soil is heated to very low temperature (200- 1000 °F) to increase the
vaporization and separation of low boiling point contaminants from the soil. By
this process organic contaminants can be completely or partially decomposed
depending upon the thermal stripping temperature and organic compounds present
in the soil. [22]. This method can remove approximately 90% of the contaminants
but it is very costly and not eco-friendly.
Another
way to remove crude oil from the soil is incineration. The contaminated soil is
burned by using fire at high temperature (1600-2500 °F) [1]. This method is
also not environmentally friendly as volatile and flammable compounds present
in crude oil will cause the environment pollution.
Biological
Methods
Bioremediation
is a traditional method that involves the use of living organisms (bacteria,
fungi and plants) to degrade harmful substances present in the environment.
Bioremediation of crude oil from the soil is very efficient, cheap and
environmentally friendly solution. The effectiveness of this method is depended
on hydrocarbon concentration, soil characteristics and composition of pollutants
[8].
PAH
are the most resistant and toxic group of soil pollutants present in the crude
oil. PAH get trapped in the soil pores after they enter into the soil and
retained by the soil matrix. So, their removal from the soil is very difficult
[23]. Bioremediation is the most suitable method to remove PAH from the soil as
microbes and plant roots can access these tiny pores easily.
Microbe
assisted remediation
Soil
is a diverse ecosystem as it inhabits various microbial populations. The
composition of naturally residing microbes change with the composition and
concentration of contaminants, so only resistant consortium of microbes
survives and work actively in the cleaning of polluted soil [24]. Hydrocarbon
degrading microbes are extensively present naturally in the contaminated soil
and breakdown complex hydrocarbons into simple form by the use of their
enzymatic systems.
Different
bacterial genera chose different types of hydrocarbons for the degradation
(Table 2) and they can also work in both aerobic and anaerobic condition. In
anaerobic condition, bacteria present in the deepest parts of the sediments use
nitrates, sulfates and iron as electron acceptor to degrade the hydrocarbons.
Some of the species of anaerobic bacteria belonging to genus Desulfococcus,
Thauera, Dechloromonas and Azoarcus exhibit hydrocarbon degradation ability
[25-26].
While
in aerobic condition, bacterial dioxygenase enzymes incorporate oxygen into
carbon molecule through a series of enzyme catalyzed reactions to generate
hydrocarbon with alcohol group. Alcohol groups are oxidized to aldehyde and
then converted into carboxylic group by the action of other enzymes which in
turn is degraded to acetyl co-A by beta oxidation [27].
The
major bacterial genera that showed crude oil degrading capability are
Alcaligenes, Sphingomonas, Pseudomonas, Bacillus, Nocardia, Acinetobacter,
Micrococcus, Achromobacter, Rhodococcus, Alcaligenes, Moraxella, Mycobacterium,
Aeromonas, Xanthomonas, Athrobacter, Flavobacterium, Micrococcus, zospirillum
[1, 2,8,27- 30].
Fungal
mycelium is very helpful in the degradation of hydrocarbons because of their
penetration ability, it also aids in the entrance of bacteria to the deep soil.
Fungal laccase, lignin peroxidase and manganese peroxidase enzymes degrade the
hydrocarbons by its oxidation [31]. Crude oil degradation has been shown by
some members of the following fungal genera: Candida, Stropharia, Rhodotorula,
Pleurotus, Penicillium, Phanerochaete, Fusarium [8, 14, 32,27].
Microbial
remediation of contaminated soil is affected by many factors such as water
amount, temperature and pH of soil, concentration of oxygen, soil quality and
amount of nutrients. Change in any of these factors can decrease the population
of microbes and in turn decreases the bioremediation [33].
Microbial
activity can be accelerated by using bioaugmentation and bio stimulation
strategies. In bioaugmentation exogenous oil degrading bacteria are
supplemented to enhance soil microbiota while in bio stimulation addition of
nutrients, aeration and optimization of physical conditions like pH and
temperature is performed. Research has shown that bioaugmentation and bio
stimulation when used together effectively remediate crude oil hydrocarbons
polluted soil. It has been observed that the number of exogenous bacteria
decreases after sometimes because of nutrient unavailability or other abiotic
factors (pH, temperature or oxygen). So, bio stimulation incorporation with
bioaugmentation provided effective results in the degradation of crude oil
pollutants (Figure 1) [1,23,30,34-36]. Different types of surfactants produced
by many microorganisms are called biosurfactants. These biosurfactants enhance
the bioavailability of hydrocarbons to the microbes and in turn increases its
degradation. Use of biosurfactants producing microbes is a good bioremediation
choice as this process is cheap, nontoxic with efficient degradation rate. So,
researchers have turned their focus towards such microbes that can degrade
crude oil and produces biosurfactants at the same time [37].
Phytoremediation
Phytoremediation
is an effective, solar driven and low-cost strategy that uses plants for the
removal of contaminants from the soil of large contaminated area. Plants have
the ability to grow in polluted soil by metabolizing or accumulating the
harmful compounds in their roots or shoots [45].
Plants
with extended root systems, minimum water requirement, adaptability to a
variety of environmental conditions and fast growth rate are appropriate for
this purpose [46]. Phytoremediation efficiency depends on the plant species
selection, environmental conditions and rhizobacteria [47].
Analysis
of soil of the Possession Island after diesel leakage in 1997 showed that area
with vegetation has 10% low concentration of hydrocarbons as compared to
non-vegetation area [48].
Different
mechanisms are devised by plants for the removal of contaminants i.e.,
phytoaccumulation (absorption of contaminants into the roots or shoots),
phytodegradation (degradation of pollutants by utilization of plant enzymes
such as laccase, oxygenase and nitroreductase), phytovolatization (release of
volatile metabolites into the atmosphere) and phytostabilization (decrease the
movement of contaminants) [11,49,50] reported that two plant species i.e.,
Eleusine indica and Cynodon dactylon significantly eliminated some low to
medium molecular weight PAH from the soil by phytoextraction process,
indicating their use in the removal of PAH.
Maize
plants showed enhanced biodegradation in association with Cynanchum laeve. This
symbiotic relationship between maize roots and Cynanchum laeve degraded 4-6
rings PAH more efficiently than any other treatment [11].
Vetiver
grass, belongs to the Poaceae family, is a perennial grass. It decontaminates
the soil by extraction of PAH and other toxins from the soil and accumulating
it in the roots and shoots. This plant showed negative effect on its growth and
other physical activities when grown on soil contaminated with diesel [51]
Mirabilis jalapa, is also considered a good candidate for phytoremediation.
[52] investigated that M. jalapa can remove 41-63% of saturated hydrocarbons
within 127 days when compared with natural attenuation process (Figure 2).
Similarly,
ryegrass, alfalfa, tall fescue, prairie grasses, meadow fescue, yellow medick,
soybeans, Gazania, Mimosa pudica, Cyperus rotundus have shown good crude oil
remediation [53-60].
With
all the advantages, phytoremediation also has some drawbacks i.e., it is a
time-consuming process, limited remediation in high pollutants concentration
and limited area of success [47].
Rhizoremediation
(Plant-microbe assisted remediation- recent technology)
Rhizoremediation
requires such plants that can grow in oil contaminated soil and also provide
favorable environment to contaminants degrading microbes by exudates secretion
or aeration. Plant-microbe strategy not only increases the metabolic activity
of rhizosphere microbes, but it also improves the soil physical and chemical
properties and increases microbial access to the contaminants present in the
soil [56].
PAH
degrading bacterial strain Rhodococcus ruber Em1 showed enhanced degradation
rate when combined with Orychophragmus violaceus during the period of 175 days
in a controlled environment (mesocosms). The expressions of linA and RHD like
genes, coding PAH-ring hydroxylating dioxygenase, increase 3-5 times in the
mesocosoms [42]. Enhanced degradation of contaminants by maize plant was
observed when maize plant was provided with indigenous microbial biomass
inoculum [61].
Glycine
max (Soybean) plant is among those plants that exhibit hydrocarbon remediating
capability. Research showed that soybean remediation of crude oil was not
because of the phytoaccumulation but it was a mutual action of G. max and
rhizospheric microbes. It was observed that Glycine max growth in the
contaminated soil effect the total number of bacteria, amount of water, pH and
organic matter quantity [62].
A
study conducted on wheat plant in hydroponics condition showed that wheat
seedlings eliminate more than 20% of oil from the medium, but this remediating
ability enhances to 29% when grown in association with Azospirillum [63].
Bioremediation
of oil contaminated soil by using yellow alfalfa in combination with
Acinetobacter sp. strain S- 33 improved the remediation efficiency 39% in
comparison to alone alfalfa (34%) and Acinetobacter sp. S-33 (35%). Fractional
Contaminants analysis showed that plant microbe association is the most
efficient strategy in the cleanup of aromatic hydrocarbons from the soil [63].
Plant
growth promoting bacteria (PGPR) promote the tolerance and resistance of plants
against contaminants present in the soil. Ryegrass when grown with PGPR showed
increased degradation of hydrocarbons to 61.5% for 3 years when 13% TPH content
was used. It was observed that low concentration enhanced the degradation and
vice versa [3,64].
Crude
oil after leakage gets trapped or physically bound with the soil particles;
access to these micro spaces is made possible by plant roots. Roots of plants
harbor microbes in the rhizosphere as well as on the surface. So, root
generates a pathway for these microbes to have access to these contaminants.
Once in the soil micropores, GPR increases the solubility of oil droplets by
producing biosurfactants or by adhering to the surface of the oil droplets.
Microbial surface membrane oxygenase’s than generate fatty acid analogues by
adding oxygen atoms into PHC. In this way microbes keep on growing and
degrading contaminants. Tentatively, microbes use 150mg of nitrogen and 30mg of
potassium to degrade 1g of PHC [65]. Utilization of plants and microbes in
collaboration is indeed a good strategy to recovery contaminated soil. It might
be a long process, but it is safer and environment friendly. Further field
experiments must be performed to develop good models.
Conclusion
Crude
oil is a quick and easily accessible energy source found in most of the
countries. Its leakage during extraction and transportation has posed danger to
the environment because it contains mutagenic and carcinogenic compounds. Soil
contamination due to crude oil leakage has adverse effects on human and
vegetation growth so its removal is essential. Many methods have been developed
to remove crude oil from the soil i.e., physical, chemical, thermal and
biological. Many alterations and development have been introduced in
Physio-chemical and thermal methods to enhance their efficiency and reduce
their demerits. Still these methods have many drawbacks and less acceptable by
the society. On the other hand, bioremediation methods are preferred because
they are efficient, cheap and nature friendly. In the recent technology i.e.,
rhizoremediation, microbes and plants are combined together in synergistic
relationship to efficiently remove the crude oil contaminants from the soil.
Research has shown that rhizoremediation is more efficient than microbial and
phytoremediation techniques separately.
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